Gas sensor

ABSTRACT

A gas sensor  10  includes a tubular protective cover  30  having an element chamber  37  therein and configured to allow a measured gas to flow from the outside into the element chamber  37 . The gas sensor  10  also includes a long sensor element  20 , including a detecting portion  23  located in the element chamber  37  and configured to detect a specified gas concentration in the measured gas, and an insulator  45  having an inclined through-hole into which the sensor element  20  is inserted. An inclination angle θt of an axial direction of the sensor element  20  (i.e., a direction parallel to an element axis A 1 ) in the element chamber  37  with respect to an axial direction of the protective cover  30  (i.e., a direction parallel to a cover axis A 2 ) is 1° or greater.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. patent application Ser. No.15/248,160, filed on Aug. 26, 2016, which claims priority under 35U.S.C. § 119(a) to Japanese Patent Application No. 2015-191249, filed inJapan on Sep. 29, 2015. The entire contents of U.S. patent applicationSer. No. 15/248,160 and the Japanese Patent Application No. 2015-191249are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a gas sensor.

2. Description of the Related Art

Gas sensors have been known, which detect the concentration of aspecified gas, such as NOx, in a gas to be measured (hereinafterreferred to as a measured gas), such as an automobile exhaust gas. Forexample, Patent Literature (PTL) 1 describes a gas sensor that includesa sensor element secured to a tubular housing, and a tubularmeasured-gas-side cover (protective cover) configured to cover a distalend of the sensor element.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 3932881

SUMMARY OF THE INVENTION

In this gas sensor, when a measured gas circulates inside the protectivecover and reaches a detecting portion of the sensor element, thedetecting portion detects a specified gas concentration. Due to thelength of time required for the measured gas to reach the detectingportion, responsiveness in detecting the specified gas concentration maybe low.

The present invention has been made to solve the problem describedabove. A primary object of the present invention is to improveresponsiveness in detecting a specified gas concentration.

The present inventors carried out studies and predicted that because theflow of the measured gas in an inner space of the protective coverhaving the sensor element therein was laminar, the measured gas couldnot easily reach the detecting portion inside, or on the surface of, thedistal end of the sensor element and thus the responsiveness describedabove was lowered. As a result of further studies, the present inventorsfound that positioning the sensor element of the gas sensor of therelated art on the axial line of the protective cover tended to causethe flow of the measured gas to be laminar. The present inventors thendiscovered that, by inclining the sensor element from the axialdirection of the protective cover, responsiveness in detecting aspecified gas concentration could be improved. This led to thecompletion of the present invention.

That is, the present invention adopts the following measures to achievethe primary object described above.

A gas sensor of the present invention comprises:

a tubular protective cover having an element chamber therein andconfigured to allow a measured gas to flow from the outside into theelement chamber; and

a long sensor element including a detecting portion located in theelement chamber and configured to detect a specified gas concentrationin the measured gas,

wherein an inclination angle θt of an axial direction of the sensorelement in the element chamber with respect to an axial direction of theprotective cover is 1° or greater.

In this gas sensor, the axial direction of the sensor element in theelement chamber is inclined with respect to the axial direction of theprotective cover, and the inclination angle θt is 1° or greater. Thus,the sensor element can disturb the flow of the measured gas in theelement chamber, and this makes the flow of the measured gas less likelyto be laminar (i.e., more likely to be turbulent). Therefore, themeasured gas can more easily reach the detecting portion, and theresponsiveness of the gas sensor in detecting a specified gasconcentration can be improved.

In the gas sensor according to the present invention, the inclinationangle θt may be 3° or greater. In this case, the occurrence of laminarflow of the measured gas can be more effectively reduced by theinclination of the sensor element, and the responsiveness of the gassensor can be further improved.

In the gas sensor according to the present invention, the inclinationangle θt may be 15° or less. In this case, an increase in pressure lossin the element chamber caused by the inclination of the sensor elementcan be reduced. It is thus possible to reduce slowing down of the flowof the measured gas caused by the pressure loss, and reduce degradationin the responsiveness of the gas sensor.

The gas sensor according to the present invention may further include aprotective layer having a greater thickness at a portion covering asurface of the sensor element on an inclination side in the elementchamber than at a portion covering a surface of the sensor elementopposite the inclination side in the element chamber. Since thisunevenness in the thickness of the protective layer disturbs the flow ofthe measured gas in the element chamber and makes it less likely to belaminar, the responsiveness of the gas sensor can be further improved.

In the gas sensor according to the present invention, the protectivecover may have an element chamber inlet open to the element chamber andserving as a passage of the measured gas; and an element chamber outletopen to the element chamber, serving as a passage of the measured gas,and located closer to a distal end of the protective cover than theelement chamber inlet is. Generally, when the element chamber inlet andthe element chamber outlet have this positional relationship, the flowof the measured gas tends to be a laminar flow along the axial directionof the protective cover, and this tends to lower the responsiveness ofthe gas sensor. Therefore, it is of great significance to apply thepresent invention.

In the gas sensor according to the present invention, the sensor elementmay have a plate-like shape and a portion of the sensor element, theportion being located in the element chamber, may be inclined in athickness direction of the sensor element with respect to the axialdirection of the protective cover. In this case, for example, ascompared to the case where the sensor element is inclined in a directionperpendicular to the thickness direction, the area of a surface of thesensor element intersecting the axial direction of the protective coveris large. This disturbs the flow of the measured gas in the elementchamber, makes it less likely to be laminar, and further improves theresponsiveness of the gas sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating how a gas sensor 10 isattached to a pipe 90.

FIG. 2 is a longitudinal cross-sectional view of the gas sensor 10.

FIG. 3 is a cross-sectional view perpendicular to a width direction of asensor element 20.

FIG. 4 is a diagram for explaining an axial direction of the sensorelement 20 with warpage in the element chamber 37.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described withreference to the drawings. FIG. 1 is a schematic diagram illustratinghow a gas sensor 10 according to an embodiment of the present inventionis attached to a pipe 90. FIG. 2 is a longitudinal cross-sectional viewof the gas sensor 10 taken along line A-A of FIG. 1. A frame indicatedby a two-dot chain line in the lower right part of FIG. 2 provides aschematic perspective view of a sensor element 20. FIG. 3 is across-sectional view perpendicular to a width direction of the sensorelement 20. In the present embodiment, the up-down direction and theright-left direction are as shown in FIG. 2.

As illustrated in FIG. 1, the gas sensor 10 is attached to the pipe 90,such as a vehicle exhaust gas pipe, and used to measure a specified gasconcentration, which is the concentration of a specified gas, such asNOx or O₂, contained in an exhaust gas serving as a measured gas. In thepresent embodiment, the gas sensor 10 is used to measure a NOxconcentration as a specified gas concentration.

As illustrated in FIG. 2, the gas sensor 10 includes the sensor element20; a protective layer 29 configured to cover at least part of thesurface of the sensor element 20; a protective cover 30 configured toprotect a distal portion (lower end portion in FIG. 2), including adistal face 20 a, of the sensor element 20; an element sealing unit 40configured to seal and secure the sensor element 20; and an assembledunit 50 configured to protect a proximal portion (upper end portion inFIG. 2), including a proximal face 20 b, of the sensor element 20 andextract an electric signal from the sensor element 20.

The sensor element 20 has a laminated structure with a plurality ofoxygen ion-conducting solid electrolyte layers, such as zirconia (ZrO₂)layers. The sensor element 20 is a long plate-like element (in the shapeof a rectangular parallelepiped) having the distal face 20 a at thelower end thereof, the proximal face 20 b at the upper end thereof, andfirst to fourth faces 21 a to 21 d perpendicular to the distal face 20 aand the proximal face 20 b. In the sensor element 20, a direction alongthe longest of the three sides of the rectangular parallelepiped (e.g.,a side shared by the first face 21 a and the fourth face 21 d) isdefined as a length direction, a direction along the shortest of thethree sides of the rectangular parallelepiped (e.g., a side shared bythe distal face 20 a and the fourth face 21 d) is defined as a thicknessdirection, and a direction along the remaining one of the three sides ofthe rectangular parallelepiped (e.g., a side shared by the distal face20 a and the first face 21 a) is defined as a width direction. The axialdirection of the sensor element 20 is inclined by an inclination angleθt with respect to the axial direction of the protective cover 30. Theinclination angle θt will be described in detail later on. The sensorelement 20 may be, for example, 25 mm to 100 mm long, 2 mm to 10 mmwide, and 0.5 mm to 5 mm thick. As illustrated in FIGS. 2 and 3, thesensor element 20 has a measured gas introduction port 22 a forintroducing therein the measured gas, a reference gas introduction port22 b for introducing therein a reference gas (atmosphere) serving as areference for detection of a specified gas concentration, and adetecting portion 23 configured to detect a specified gas concentrationin the measured gas. The measured gas introduction port 22 a is open inthe distal face 20 a of the sensor element 20, and is located in anelement chamber 37 which is a space inside an inner protective cover 31.The reference gas introduction port 22 b is open in the proximal face 20b of the sensor element 20 as illustrated in FIG. 3, and is located in aspace inside an atmosphere-side cover 74.

The detecting portion 23 includes at least one electrode, and isdisposed in the element chamber 37 to detect a specified gasconcentration in the measured gas in the element chamber 37. In thepresent embodiment, the detecting portion 23 includes an outer electrode24 disposed on the first face 21 a; and inner main pump electrodes 25,inner auxiliary pump electrodes 26, a measurement electrode 27, and areference electrode 28 disposed inside the sensor element 20. Themeasured gas in the element chamber 37 reaches the outer electrode 24and the measured gas introduction port 22 a. The measured gas introducedthrough the measured gas introduction port 22 a into the inside of thesensor element 20 reaches the inner main pump electrodes 25, the innerauxiliary pump electrodes 26, and the measurement electrode 27 in thisorder. The reference gas introduced through the reference gasintroduction port 22 b reaches the reference electrode 28. The principleon the basis of which the detecting portion 23 detects a specified gasconcentration will not be described in detail, as it is well known. Forexample, the detecting portion 23 detects a specified gas concentrationin the following manner. On the basis of a voltage applied between theouter electrode 24 and the inner main pump electrodes 25, the detectingportion 23 pumps oxygen in the measured gas around the inner main pumpelectrodes 25 into or out of the detecting portion 23 (to the elementchamber 37). Also, on the basis of a voltage applied between the outerelectrode 24 and the inner auxiliary pump electrodes 26, the detectingportion 23 pumps oxygen in the measured gas around the inner auxiliarypump electrodes 26 into or out of the detecting portion 23 (to theelement chamber 37). Thus, the measured gas having an oxygenconcentration adjusted to a predetermined value reaches the vicinity ofthe measurement electrode 27. The measurement electrode 27 serves as aNOx reduction catalyst to reduce a specified gas (NOx) in the measuredgas reached. Then, an electromotive force generated between themeasurement electrode 27 and the reference electrode 28 in accordancewith the oxygen concentration after the reduction or a current based onthe electromotive force is generated as an electric signal by thedetecting portion 23. The electric signal thus generated by thedetecting portion 23 is a signal representing a value corresponding to aspecified gas concentration in the measured gas (i.e., a value fromwhich the specified gas concentration can be derived), and is equivalentto a value detected by the detecting portion 23. The electric signal isoutput to the outside through a conductive electrode (not shown)disposed on the surface of the proximal portion of the sensor element20.

The protective layer 29 is a porous layer that covers at least part ofthe surface of the sensor element 20. The protective layer 29 covers theentire distal face 20 a of the sensor element 20 and most parts of thefirst to fourth faces 21 a to 21 d positioned in the element chamber 37.The protective layer 29 also covers the outer electrode 24 and themeasured gas introduction port 22 a, but since the protective layer 29is a porous layer, the measured gas can pass through the protectivelayer 29 to reach the outer electrode 24 and the measured gasintroduction port 22 a. The protective layer 29 serves to protect thesensor element 20 from, for example, moisture or oil components in themeasured gas. The protective layer 29 is a porous layer of ceramic, suchas alumina. The protective layer 29 has a greater thickness at a portioncovering the first face 21 a (i.e., a portion on the left side of thesensor element 20 in FIG. 2) than at a portion covering the second tofourth faces 21 b to 21 d and the distal face 20 a. Therefore, theprotective layer 29 has a greater thickness at a portion covering asurface (first face 21 a) of the sensor element 20 on the inclinationside in the element chamber 37 than at a portion covering a surface(second face 21 b) of the sensor element 20 opposite the inclinationside in the element chamber 37.

The protective cover 30 is a cylindrical member made of metal, such asstainless steel, and configured to allow the measured gas to flow fromoutside into the element chamber 37. The protective cover 30 includesthe inner protective cover 31 which is a cylindrical member with abottom and configured to cover the distal portion of the sensor element20, and an outer protective cover 32 which is a cylindrical member witha bottom and configured to cover the inner protective cover 31. Theinner protective cover 31 has the element chamber 37 therein. Theelement chamber 37 is a space surrounded by the inner periphery of theinner protective cover 31. The distal portion of the sensor element 20,including the distal face 20 a and the detecting portion 23, is disposedin the element chamber 37. The inner protective cover 31 and the outerprotective cover 32 are provided with a gas chamber 38 therebetween,which is a space surrounded by the inner protective cover 31 and theouter protective cover 32.

The inner protective cover 31 has a plurality of element chamber inlets33 in the periphery thereof, and an element chamber outlet 34 at thebottom thereof. The element chamber inlets 33 and the element chamberoutlet 34 are open to the element chamber 37 and the gas chamber 38. Theelement chamber inlets 33 serve as passages of the measured gas from thegas chamber 38 to the element chamber 37. The element chamber outlet 34serves as a passage of the measured gas from the element chamber 37 tothe gas chamber 38. The element chamber inlets 33 are located closer tothe proximal face 20 b than the distal face 20 a (and the measured gasintroduction port 22 a) of the sensor element 20 is (i.e., located abovethe distal face 20 a). At the same time, the element chamber inlets 33are located above the outer electrode 24, which is an electrode of thedetecting portion 23 and is disposed on the surface of the sensorelement 20. The element chamber outlet 34 is located below the elementchamber inlets 33, that is, closer to the distal end of the protectivecover 30 than the element chamber inlets 33 are. At the same time, theelement chamber outlet 34 is located below the distal face 20 a of thesensor element 20. The element chamber inlets 33 and the element chamberoutlet 34 are circular holes. The inside diameter of the innerprotective cover 31 may be, for example, from 5 mm to 10 mm. Thevertical distance between the element chamber inlets 33 and the elementchamber outlet 34 (i.e., the distance in the axial direction of theinner protective cover 31) may be, for example, from 10 mm to 20 mm.

The outer protective cover 32 has a plurality of outer inlets 35 in theperiphery thereof, and an outer outlet 36 at the bottom thereof. Theouter inlets 35 and the outer outlet 36 are open to the gas chamber 38and the outside of the outer protective cover 32 (i.e., inside of thepipe 90). The outer inlets 35 serve as passages of the measured gas fromthe outside to the gas chamber 38. The outer outlet 36 serves as apassage of the measured gas from the gas chamber 38 to the outside. Theouter inlets 35 are located below the element chamber inlets 33. Theouter outlet 36 is located below the element chamber outlet 34 and theouter inlets 35. The outer inlets 35 and the outer outlet 36 arecircular holes. The inside diameter of the outer protective cover 32 maybe, for example, from 7 mm to 15 mm.

The element sealing unit 40 includes a housing 41, a first insulator 45,a sealing member 48, and a seal material 49. The sensor element 20 ispositioned around the central axis of the element sealing unit 40, andvertically passes through the element sealing unit 40. The housing 41 isa cylindrical metal member with a lower end to which the upper end ofthe protective cover 30 is attached. The housing 41 is welded to thepipe 90 and inserted into a securing member 91 having female threads onthe inner periphery thereof. Thus, the gas sensor 10 is secured to thepipe 90, with the distal portion of the sensor element 20 and theprotective cover 30 of the gas sensor 10 protruding into the pipe 90.The first insulator 45 is a columnar member disposed inside the housing41. The sensor element 20 passes through the inside of the firstinsulator 45. The first insulator 45 is made of an insulating ceramic,such as alumina, steatite, or zirconia. The sealing member 48 is aring-shaped member made of metal, such as stainless steel, nickel, orcopper. The sealing member 48 is pushed upward and downward by thehousing 41 and the first insulator 45, respectively, and configured tohermetically seal the gap between the housing 41 and the first insulator45. The seal material 49 is obtained by molding ceramic powder, such astalc, alumina, or boron nitride powder. The seal material 49 is chargedinto the space between the inner periphery of the first insulator 45 andthe sensor element 20, whereby the space between the first insulator 45and the sensor element 20 is hermetically sealed. The first insulator 45and the seal material 49 each have an inclined through-hole, into whichthe sensor element 20 is inserted and secured in place at theinclination angle θt.

The assembled unit 50 includes a second insulator 55, a plurality ofcontact fittings 60, a plurality of connection terminals 71, a pluralityof lead wires 72, a rubber stopper 73, an atmosphere-side cover 74, anouter cover 75, and a disc spring 77. The second insulator 55 is acylindrical member with a bottom, and is made of an insulating ceramicas in the case of the first insulator 45. The second insulator 55 is incontact, on the lower face thereof, with the upper face of the firstinsulator 45, and is positioned to be coaxial with the first insulator45. The contact fittings 60 are metal plates each bent at multiplepoints and having a plate spring portion folded inward. A plurality of(e.g., two or three) contact fittings 60 are disposed on each of theright and left sides of the sensor element 20, and connected tocorresponding conductive electrodes (not shown) disposed on the firstand second faces 21 a and 21 b of the sensor element 20. The platespring portions of the contact fittings 60 are supported by the secondinsulator 55 by a reactive force against their elastic force, and areelectrically connected to the conductive electrodes on the sensorelement 20 by pushing the sensor element 20 from both the right and leftsides. An end portion of each of the contact fittings 60 passes througha hole on the upper side of the second insulator 55 to protrudetherefrom and is electrically connected to the corresponding lead wire72, with the corresponding connection terminal 71 interposedtherebetween. The lead wires 72 vertically pass through the rubberstopper 73 that closes the upper end of the atmosphere-side cover 74 andthe outer cover 75, and extend to the outside (i.e., to the outside ofthe pipe 90, or to the atmosphere). The atmosphere-side cover 74 coversa part of the outer periphery of the element sealing unit 40 on theproximal side (upper side) of the sensor element 20. At the same time,the atmosphere-side cover 74 covers the second insulator 55 and therubber stopper 73. The outer cover 75 covers the outer periphery of theupper side of the atmosphere-side cover 74. The atmosphere-side cover 74and the outer cover 75 each have a plurality of atmosphere introductionholes 76, through which the reference gas (atmosphere) passes and isintroduced through the reference gas introduction port 22 b into thesensor element 20. The atmosphere-side cover 74 and the outer cover 75have a swaged portion with a reduced diameter near the upper endthereof. With this swaged portion, the rubber stopper 73 is secured inplace. The disc spring 77 is sandwiched between the atmosphere-sidecover 74 above and the second insulator 55 below. By pushing the secondinsulator 55 downward, the disc spring 77 secures the second insulator55 and the first insulator 45 in place.

The inclination angle θt of the sensor element 20 will now be describedin detail. The inclination angle θt is the angle of inclination of theaxial direction of the sensor element 20 in the element chamber 37 withrespect to the axial direction of the protective cover 30. FIG. 2 showsan element axis A1 which is the central axis of the sensor element 20.The element axis A1 is an axis parallel to the length direction of thesensor element 20, and a direction parallel to the element axis A1 isthe axial direction of the sensor element 20. In the present embodiment,the sensor element 20 has no warpage, and a portion of the sensorelement 20 located in the element chamber 37 has the same axialdirection as the other portion (e.g., the proximal portion including theproximal face 20 b) not located in the element chamber 37. Therefore,like the axial direction of the entire sensor element 20, the axialdirection of the sensor element 20 in the element chamber 37 (i.e., theaxial direction of the portion of the sensor element 20 located in theelement chamber 37) is parallel to the element axis A1. Also in thepresent embodiment, the central axis of the gas sensor 10 coincides withthe central axis of the protective cover 30 (i.e., the inner protectivecover 31 and the outer protective cover 32), and is indicated as a coveraxis A2 in FIG. 2. The cover axis A2 is parallel to the up-downdirection in FIG. 2, and this up-down direction is the axial directionof the protective cover 30. The axial direction of the sensor element 20(i.e., a direction parallel to the element axis A1) is inclined by theinclination angle θt with respect to the axial direction of theprotective cover 30 (i.e., a direction parallel to the cover axis A2).The inclination angle θt is 1° or greater. The inclination angle θt maybe 20° or less. The sensor element 20 is inclined in the thicknessdirection of the sensor element 20. That is, as shown in the frameindicated by a two-dot chain line in FIG. 2, the sensor element 20pivots from a position in which the element axis A1 is parallel to thecover axis A2 (i.e., the position of the sensor element 20 indicated bya broken line) in the thickness direction (parallel to the distal face20 a, the third face 21 c, and the fourth face 21 d). In the presentembodiment, the sensor element 20 is inclined in the thicknessdirection, particularly in the direction in which the distal portion ofthe sensor element 20 is moved from the position of the second face 21 btoward the first face 21 a. Alternatively, the sensor element 20 may beinclined in the thickness direction, particularly in the direction inwhich the distal portion of the sensor element 20 is moved from theposition of the first face 21 a toward the second face 21 b. Note thatthe element axis A1 and the cover axis A2 do not need to intersect eachother. Even when the element axis A1 and the cover axis A2 do notintersect each other, the inclination angle θt can be determined as longas the axial direction of the protective cover 30 (i.e., a directionparallel to the cover axis A2) and the axial direction of the sensorelement 20 in the element chamber 37 (i.e., a direction parallel to theelement axis A1) are inclined with respect to each other.

An example of using the gas sensor 10 configured as described above willnow be described. When the measured gas flows in the pipe 90, with thegas sensor 10 attached to the pipe 90 as illustrated in FIGS. 1 and 2,the measured gas circulates inside the protective cover 30.Specifically, the measured gas in the pipe 90 flows through the outerinlets 35 into the gas chamber 38, from which it flows through theelement chamber inlets 33 into the element chamber 37. In the elementchamber 37, the measured gas flows from the element chamber inlets 33toward the element chamber outlet 34, from which it flows through theouter outlet 36 to the outside (i.e., to the inside of the pipe 90). Inthe present embodiment, the protective cover 30 is of a so-calledaspirator type (which may also be referred to as a differential-pressureaspiration type). That is, by using a phenomenon in which a staticpressure in the vicinity of the central axis of the pipe 90 is lowerthan that in the vicinity of the inner periphery of the pipe 90, theprotective cover 30 is configured such that the measured gas flows inthrough the outer inlets 35 in the vicinity of the inner periphery ofthe pipe 90 and flows out through the outer outlet 36 in the vicinity ofthe central axis of the pipe 90. Then, when the measured gas reaches thedetecting portion 23 during its circulation in the element chamber 37,or more specifically, when the measured gas reaches the outer electrode24 and also reaches the inside of the sensor element 20 through themeasured gas introduction port 22 a, the detecting portion 23 generatesan electric signal corresponding to a specified gas concentration in themeasured gas as described above. By extracting the electric signalthrough the contact fittings 60, the connection terminals 71, and thelead wires 72, the specified gas concentration is detected on the basisof the electric signal.

If the central axis of the sensor element coincides with the centralaxis of the protective cover as in the gas sensor of the related art(i.e., if the inclination angle θt is 0°), the flow of the measured gasin the element chamber tends to be laminar. The more laminar the flow ofthe measured gas, the more time it takes for the measured gas flowingaround the sensor element to reach the detecting portion at the distalend of the sensor element. When the measured gas cannot easily reach thedetecting portion, responsiveness in detecting the specified gasconcentration tends to be lowered. That is, for example, even if thespecified gas concentration in the measured gas changes, thecorresponding electric signal generated by the sensor element cannot bechanged immediately. However, in the gas sensor 10 of the presentembodiment in which the inclination angle θt is 1° or greater, thesensor element 20 in the element chamber 37 can disturb the flow of themeasured gas (i.e., downward flow from the element chamber inlets 33toward the element chamber outlet 34 in the present embodiment). Thismakes the flow of the measured gas in the element chamber 37 less likelyto be laminar (i.e., more likely to be turbulent). Therefore, themeasured gas flowing around the sensor element 20 can more easily reachthe detecting portion 23 at the distal end of the sensor element 20, andthe responsiveness of the gas sensor 10 in detecting the specified gasconcentration can be improved.

The inclination angle θt is preferably 3° or greater. In this case, theoccurrence of laminar flow of the measured gas can be more effectivelyreduced by the inclination of the sensor element 20, and theresponsiveness of the gas sensor 10 can be further improved. Theinclination angle θt is more preferably 5° or greater, and still morepreferably 9° or greater. At the same time, the inclination angle θt ispreferably 15° or less. In this case, an increase in pressure loss inthe element chamber 37 caused by the inclination of the sensor element20 can be reduced. It is thus possible to reduce slowing down of theflow of the measured gas caused by the pressure loss, and reducedegradation in the responsiveness of the gas sensor 10. The inclinationangle θt is more preferably 13° or less, and still more preferably 11°or less. As in the present embodiment, it is preferable that thedirection of inclination of the sensor element 20 in the element chamber37 be opposite the direction of flow of the measured gas (i.e., fromleft to right in FIG. 2). Also, as in the present embodiment, it ispreferable that the direction of inclination of the sensor element 20 inthe element chamber 37 be a direction toward one of the element chamberinlets 33.

In the gas sensor 10 of the present embodiment described above indetail, where the inclination angle θt is 1° or greater, the measuredgas can easily reach the detecting portion 23 of the sensor element 20and the responsiveness of the gas sensor 10 in detecting the specifiedgas concentration is improved. If the inclination angle θt is 3° orgreater, the responsiveness of the gas sensor 10 is further improved. Atthe same time, if the inclination angle θt is 15° or less, degradationin the responsiveness of the gas sensor 10 caused by an increase inpressure loss can be reduced. Generally, if the measured gas has atleast one of a low flow velocity, a low flow rate, and a low pressure,it is particularly difficult for the measured gas to reach the detectingportion 23. Therefore, in such a case, it is of particularly greatsignificance to reduce the occurrence of laminar flow in the elementchamber 37 by setting the inclination angle θt to 1° or greater.

The gas sensor 10 includes the protective layer 29 having a greaterthickness at a portion covering the surface (first face 21 a) of thesensor element 20 on the inclination side in the element chamber 37 thanat a portion covering the surface (second face 21 b) of the sensorelement 20 opposite the inclination side in the element chamber 37.Since this unevenness in the thickness of the protective layer 29disturbs the flow of the measured gas in the element chamber 37 andmakes it less likely to be laminar, the responsiveness of the gas sensor10 can be further improved.

Also in the gas sensor 10, the protective cover 30 has the elementchamber inlets 33 open to the element chamber 37 and serving as passagesof the measured gas; and the element chamber outlet 34 open to theelement chamber 37, serving as a passage of the measured gas, andlocated closer to the distal end of the protective cover 30 than theelement chamber inlets 33 are. Generally, when the element chamberinlets and the element chamber outlet have this positional relationship,the flow of the measured gas tends to be a laminar flow along the axialdirection of the protective cover, and this may lower the responsivenessof the gas sensor. Therefore, it is of great significance, in theprotective cover 30 configured as described above, to reduce theoccurrence of laminar flow in the element chamber 37 by setting theinclination angle θt to 1° or greater.

Also in the gas sensor 10, the sensor element 20 has a plate-like shape,and a portion of the sensor element 20 located in the element chamber 37is inclined in the thickness direction of the sensor element 20 withrespect to the axial direction of the protective cover 30. Therefore,for example, as compared to the case where the sensor element 20 isinclined in a direction (width direction) perpendicular to the thicknessdirection, the area of a surface of the sensor element 20 intersectingthe axial direction of the protective cover 30 is large. Specifically,for example, if the sensor element 20 is inclined in the widthdirection, the third face 21 c or the fourth face 21 d intersects theaxial direction of the protective cover 30, whereas if the sensorelement 20 is inclined in the thickness direction, the first face 21 a(as in the present embodiment) or the second face 21 b having a greaterarea than the third and fourth faces 21 c and 21 d intersects the axialdirection of the protective cover 30. In the latter case, since the flowof the measured gas in the element chamber 37 is disturbed and is lesslikely to be laminar, the responsiveness of the gas sensor 10 is furtherimproved.

The present invention is by no means limited to the embodimentsdescribed above, and can be carried out in various ways within thetechnical scope of the present invention.

For example, although the sensor element 20 is inclined in the thicknessdirection in the embodiments described above, the present invention isnot limited to this. That is, the sensor element 20 may be inclined in adirection (width direction) perpendicular to the thickness direction, ormay be inclined in both the thickness and width directions.

Although the central axis of the gas sensor 10 coincides with thecentral axis of the protective cover 30 (i.e., the inner protectivecover 31 and the outer protective cover 32) in the embodiments describedabove, the present invention is not limited to this. The inclinationangle θt is determined on the basis of the direction of the central axis(element axis A1) of the sensor element 20 and the direction of thecentral axis (cover axis A2) of the protective cover 30. For example,the central axis of the gas sensor 10 does not necessarily need tocoincide with, or does not necessarily need to be parallel to, thecentral axis of the protective cover 30. Also, the central axis of thegas sensor 10 may be parallel to (or may coincide with) the central axisof the sensor element 20, as long as the axial direction of the sensorelement 20 and the axial direction of the protective cover 30 areinclined with respect to each other. Although the central axis of theinner protective cover 31 coincides with that of the outer protectivecover 32 in the embodiments described above, the present invention isnot limited to this. If the axial direction of the inner protectivecover 31 differs from that of the outer protective cover 32, the axialdirection of the inner protective cover 31 (or in other words, the axialdirection of the element chamber 37) is defined as “the axial directionof the protective cover”.

In the embodiments described above, the sensor element 20 has nowarpage, and a portion of the sensor element 20 located inside theelement chamber 37 and the other portion located outside the elementchamber 37 have the same axial direction. However, the present inventionis not limited to this. That is, the different portions of the sensorelement 20 described above may have different axial directions, as longas the inclination angle θt, which is determined on the basis of theaxial direction of the portion of the sensor element 20 located in theelement chamber 37, is 1° or greater. If, for example, the sensorelement 20 has warpage in the element chamber 37 and the axial line ofthe sensor element 20 is not straight at the portion in the elementchamber 37, a direction parallel to a straight line connecting both endsof this axial line is defined as “the axial direction of the sensorelement in the element chamber”. FIG. 4 is a diagram for explaining theaxial direction of the sensor element 20 with warpage in the elementchamber 37. In FIG. 4, a straight line (broken line) connecting pointsP1 and P2 at both ends of an axial line A3 (two-dot chain line) of theportion of the sensor element 20 in the element chamber 37 is defined asthe element axis A1, and a direction parallel to the element axis A1 isdefined as the axial direction of the sensor element 20 in the elementchamber 37. An angle formed by the direction parallel to the elementaxis A1 and a direction parallel to the cover axis A2 is defined as theinclination angle θt.

In the embodiments described above, the first insulator 45 and the sealmaterial 49 each have an inclined through-hole, into which the sensorelement 20 is inserted and secured in place at the inclination angle θt.However, the present invention is not limited to this. In the gas sensor10, the sensor element 20 may be secured in any manner as long as theinclination angle θt is 1° or greater. For example, the sensor element20 may be formed in a warped state, and the first insulator 45 and theseal material 49 may each have a through-hole shaped to accommodate thiswarpage. If the sensor element 20 has no warpage, the sensor element 20may be pushed by the shape of the through-holes of the first insulator45 and the seal material 49 so that the sensor element 20 is warped (orbent) to adjust the inclination angle θt to 1° or greater.

The shape of the protective cover 30 is not limited to the embodimentsdescribed above. For example, at least one of the inner protective cover31 and the outer protective cover 32 may have a stepped portion formedby axially connecting a large-diameter portion on the proximal side to asmall-diameter portion on the distal side. The arrangement and number ofthe element chamber inlets 33, the element chamber outlet 34, the outerinlets 35, and the outer outlet 36 are not limited to those described inthe embodiments. Although the element chamber inlets 33, the elementchamber outlet 34, the outer inlets 35, and the outer outlet 36 arecircular holes in the embodiments described above, their shape is notlimited to this. That is, they may be oval holes, polygonal (e.g.,rectangular) holes, or slit-like openings, not holes. Although theprotective cover 30 is of a so-called aspirator type (which may also bereferred to as a differential-pressure aspiration type) in theembodiments described above, the type of the protective cover 30 is notparticularly limited to this. That is, the measured gas may flow in andout of the protective cover 30 without using a differential pressurebetween the vicinity of the inner periphery of the pipe 90 and thevicinity of the central axis of the pipe 90. Although the protectivecover 30 has a two-layer structure of the inner protective cover 31 andthe outer protective cover 32 in the embodiments described above, thestructure of the protective cover 30 is not limited to this. That is,the protective cover 30 may have a three or more layer structure, or mayhave a single-layer structure formed by one of the inner protectivecover 31 and the outer protective cover 32. Although the protectivecover 30 (i.e., the inner protective cover 31 and the outer protectivecover 32) is a cylindrical member in the embodiments described above,the protective cover 30 may be of any shape as long as it is tubular.

In the embodiments described above, the protective layer 29 has agreater thickness at a portion covering the surface of the sensorelement 20 on the inclination side in the element chamber 37 than at aportion covering the surface of the sensor element 20 opposite theinclination side in the element chamber 37. However, the presentinvention is not particularly limited to this. The protective layer 29covering the different faces of the sensor element 20 in the elementchamber 37 may have a uniform thickness. Although the protective layer29 covers the distal face 20 a and the first to fourth faces 21 a to 21d in the embodiments described above, at least one of these five facesdoes not necessarily need to be covered. The gas sensor 10 does notnecessarily need to include the protective layer 29. Note that “theprotective layer 29 has a greater thickness at a portion covering thesurface of the sensor element 20 on the inclination side in the elementchamber 37 than at a portion covering the surface of the sensor element20 opposite the inclination side in the element chamber 37” means thatas long as the protective layer 29 covers the surface of the sensorelement 20 on the inclination side, the protective layer 29 does notnecessarily need to cover the surface of the sensor element 20 oppositethe inclination side.

Although the measured gas introduction port 22 a is provided in thedistal face 20 a of the sensor element 20 in the embodiments describedabove, the measured gas introduction port 22 a may be provided inanother face of the sensor element 20 as long as it is located in theelement chamber 37. For example, the measured gas introduction port 22 amay be provided in the first face 21 a. This also has the effect ofimproving the responsiveness of the gas sensor 10 as long as theinclination angle θt is 1° or greater. If the measured gas introductionport 22 a is provided in the first face 21 a, the element chamber inlets33 in the inner protective cover 31 may be located above the measuredgas introduction port 22 a (i.e., located closer to the proximal face 20b of the sensor element 20 than the measured gas introduction port 22 ais).

The configuration of the detecting portion 23 is not limited to that inthe embodiments described above. The number and arrangement ofelectrodes included in the detecting portion 23 may differ from those inthe embodiments described above. For example, the detecting portion 23does not necessarily need to include the inner auxiliary pump electrodes26. Although a part of the detecting portion 23 (i.e., the inner mainpump electrodes 25, the inner auxiliary pump electrodes 26, and themeasurement electrode 27) is disposed in the interior of the sensorelement 20 extending from the measured gas introduction port 22 a in theembodiments described above, the present invention is not limited tothis. For example, the sensor element 20 does not necessarily need tohave the measured gas introduction port 22 a. In this case, for example,the detecting portion 23 may include at least the measurement electrode27 and the reference electrode 28, and the measurement electrode 27 maybe disposed on a surface (e.g., first face 21 a) of the sensor element20.

Although the sensor element 20 has a long plate-like shape in theembodiments described above, the sensor element 20 may be of any shapeas long as it is a long element. For example, the sensor element 20 mayhave a columnar shape.

EXAMPLES

Gas sensors that were specifically made will now be described asexamples. Experimental examples 2 to 10 correspond to examples of thepresent invention, and Experimental example 1 corresponds to acomparative example. The present invention is not limited to theexamples described below.

Experimental Examples 1 to 10

The gas sensors (Experimental examples 1 to 10) having the sameconfiguration as the gas sensor 10 in FIGS. 2 and 3, except for theirinclination angles θt (see Table 1) and having no protective layer 29,were made. The inside diameter of the inner protective cover 31 was 8mm. The element chamber inlets 33 were circular in shape with a diameterof 2 mm, and six element chamber inlets 33 were formed in the outerperiphery of the inner protective cover 31 at regular intervals. Theelement chamber outlet 34 was circular in shape with a diameter of 2 mm.The inside diameter of the outer protective cover 32 was 12 mm. Theouter inlets 35 were circular in shape with a diameter of 3 mm, and sixouter inlets 35 were formed in the outer periphery of the outerprotective cover 32 at regular intervals. The outer outlet 36 wascircular in shape with a diameter of 3 mm. As in FIG. 2, each of thesensor elements 20 in Experimental examples 2 to 10 was inclined in thedirection in which the distal portion of the sensor element 20 was movedfrom the position of the second face 21 b toward the first face 21 a.

(Responsiveness Test)

For the gas sensors of Experimental examples 1 to 10, the responsivenessof each sensor element 20 in detecting a specified gas concentration wasevaluated. Specifically, the evaluation was made in the followingmanner. First, the gas sensor of Experimental example 1 was attached toa pipe as in FIG. 1. The pipe was 50 mm in diameter. The length of aportion of the protective cover 30 protruding into the pipe (i.e.,length in the vertical direction in FIG. 2) was 28 mm. Air was allowedto flow into the pipe at a flow velocity of 5 msec and stand forsufficient time. The gas flowing in the pipe was then switched from airto the measured gas. The measured gas, which was nitrogen containing 500ppm of NO, was allowed to flow at a flow velocity of 5 m/sec. Thedirection of flow of the air and measured gas in Experimental example 1was from left to right in FIG. 2. The change in the output of the sensorelement 20 (i.e., an electric signal generated by the detecting portion23) with respect to time in this case was examined. With the output ofthe sensor element 20 during flow of air in the pipe being taken as 0%and the maximum output of the sensor element 20 during flow of measuredgas in the pipe being taken as 100%, the time elapsed from when theoutput exceeded 10% to when it exceeded 90% was measured and defined asa response time (msec) in detecting a specified gas concentration. Ashorter response time means higher responsiveness of the gas sensor indetecting a specified gas concentration. For Experimental examples 2 to10, the response time was measured in the same manner as above. Theinclination angle θt and the response time measured in the test for eachof Experimental examples 1 to 10 are shown in Table 1.

TABLE 1 Inclination Response angle θ t[°] time [msec] Experimentalexample 1 0 400 Experimental example 2 1 350 Experimental example 3 3320 Experimental example 4 5 300 Experimental example 5 7 250Experimental example 6 9 200 Experimental example 7 11 200 Experimentalexample 8 13 250 Experimental example 9 15 300 Experimental example 1017 350

As in Table 1, the response time in Experimental example 2 where theinclination angle θt was 1° was shorter than that in Experimentalexample 1 where the inclination angle θt was 0°. The response time had atendency to decrease as the inclination angle θt increased from 1°.Thus, the result indicates that the inclination angle θt is preferably3° or greater, more preferably 5° or greater, and still more preferably9° or greater. Table 1 also shows that the response time had a tendencyto increase when the inclination angle θt was too large. This indicatesthat to reduce an increase in response time, the inclination angle θt ispreferably 15° or less, more preferably 13° or less, and still morepreferably 11° or less. The response time was shortest when theinclination angle θt was greater than 7° and less than 13°.

The present application claims priority of Japanese Patent ApplicationNo. 2015-191249 filed on Sep. 29, 2015, the entire contents of which areincorporated herein by reference.

What is claimed is:
 1. A gas sensor comprising: a tubular protectivecover having an element chamber therein and configured to allow ameasured gas to flow from the outside into the element chamber; a longsensor element including a detecting portion located in the elementchamber and configured to detect a specified gas concentration in themeasured gas; an insulator having a first inclined through-hole intowhich the sensor element is inserted; and a seal material disposedbetween the insulator and the sensor element, the seal material having asecond inclined through-hole into which the sensor element is inserted,wherein the sensor element is disposed in the first inclinedthrough-hole and the second inclined through-hole such that aninclination angle θt of an axial direction of the sensor element in theelement chamber with respect to an axial direction of the protectivecover is greater than or equal to 1° and less than or equal to 20°, andwherein the first and second inclined through-holes are inclined at theinclination angle θt.
 2. The gas sensor according to claim 1, whereinthe inclination angle θt of the sensor element is 3° or greater.
 3. Thegas sensor according to claim 1, wherein the inclination angle θt of thesensor element is 15° or less.
 4. The gas sensor according to claim 1,further comprising a protective layer having a greater thickness at aportion covering a surface of the sensor element on an inclination sidein the element chamber than at a portion covering a surface of thesensor element opposite the inclination side in the element chamber. 5.The gas sensor according to claim 1, wherein the protective cover has anelement chamber inlet open to the element chamber and serving as apassage of the measured gas; and an element chamber outlet open to theelement chamber, serving as a passage of the measured gas, and locatedcloser to a distal end of the protective cover than the element chamberinlet is.
 6. The gas sensor according to claim 1, wherein the sensorelement has a plate-like shape and a portion of the sensor element, theportion being located in the element chamber, is inclined in a thicknessdirection of the sensor element with respect to the axial direction ofthe protective cover.
 7. The gas sensor according to claim 1, whereinthe sensor element is inserted into the insulator and secured in placeat the inclination angle θt.